1. Field of the Invention
The present invention relates to a printing head for use as an output device of a computer or the like and for use with a facsimile or the like, and in particular, to a driving circuit for driving printing elements of a thermal printer, a light-emitting diode (LED) array head of an LED printer, or the like in which a control is effected to obtain a uniform output energy from the printing elements.
2. Description of the Prior Art
In general, in a thermal printer using a thermal head described above, a variation in the resistance values of heat radiating resistor elements constituting printing elements of a printing head primarily causes an undesirable printing spot; whereas in an LED printer using the LED array head above, a fluctuation in optical outputs from light-emitting elements such as LED's constituting printing elements mainly causes the undesirable printing spot. To overcome such a problem, there have been proposed various ideas to implement a uniform density in characters and images to be printed, thereby attaining a high-quality printout product. For example, the time to supply power to the respective elements and the electric current thus supplied thereto are controlled such that a uniform temperature is developed by the heat radiation of each heat-radiating resistor element in the thermal printer and that a uniform photoenergy is obtained from each light-emitting element in the LED printer.
However, neither of these proposals include a configuration in which a driving circuit to drive the printing elements of the recording head, for example, a driving integrated circuit (to be abbreviated as IC hereinafter) has therein a permanent memory function for correction data. In the conventional technique, therefore, a memory circuit having a large storage capacity and a control circuit to control the memory circuit are required to be disposed at a location external with respect to the printing head including the printing elements and the driving circuit. As a representative example of a printer in which the printing head is driven by means of an external memory and an external control circuit described above, there has been a printer described in the Japanese Patent Laid-Open No. 58-78476 "Method and Apparatus of Controlling Light-Emitting Diode Array" (refer to the U.S. patent application Ser. No. 1981-292985 now U.S. Pat. No. 4,455,562 and the UK patent application GB No. 2 104 266 A entitled as "Control of a Light Emitting Diode Array"). The contents of the application concerning the method and apparatus of controlling the LED array will now be described with reference to FIGS. 1-3.
FIG. 1 is a schematic block diagram illustrating a substantial configuration of a driving circuit in a printing head comprising an LED array, in which the printing head 1 includes a printing elements 2 constituted from an LED array head and the like, the printing element 2 having as individual elements a plurality of LED elements 3 and a driving circuit 4 to drive the printing element 2 by effecting an output control of the printing element 2. The driving circuit 4 comprises a shift register 5 having a plurality of data input terminals 6 and a clock terminal 7 connected thereto by wirings, a latch circuit 8 connected via a plurality of lead wires 9 to the shift register 5 and connected via an internal wiring to a latch clock terminal 10, and a driving section 11 disposed between the latch circuit 8 and the printing element 2 for controlling the quantity of the emitted light of each of the LED elements 3 so as to drive the printing element 2. For each of the LED elements 3, four lead lines 9 (connecting the shift register 5 to the latch circuit 8) having a 1-bit signal transmission capacity are disposed. The driving section 11 comprises tri-state logic (TSL) buffers 12 each including four TSL buffer elements for each LED element 3 corresponding to the lead lines 9, a control bus 13 including lead lines 13a-13d to which the respective TSL buffer elements 12a-12d constituting the TSL buffer 12 are connected and which receives via each of terminals 14a-14d a correction signal having a time width equivalent to a binary value 8, 4, 2, or 1, and output transistors 15 for effecting a switching on/off of outputs from the LED elements 3, the switching operation being executed by use of a load resistor 16 disposed on an anode side when an output from one of the TSL buffers 12 is supplied to a base electrode of one of the output transistors 15. The TSL buffer 12 has four logic gates 12a-12d for each said transistor 15 and each of the logic gates 12a-12d is connected to one of the buses 13a-13d of the common wirings 13 for the switch selection as described above. The TSL buffer 12 is connected to the shift register 5 via the latch circuit 8 and lead lines 9 (for each printing element, four lead lines 9 are disposed). Consequently, the TSL buffer 12 posseses, like the lead lines 9, a 4-bit information transmission associated with the gates 12a-12d.
Next, a brief description will be given of the driving circuit 4 of FIG. 1. The gates 12a-12d of the TSL buffer 12 receive correction signals via through the respective buses 13a-13d of the control bus conductor 13 pulses with time width of binary values 8, 4, 2, and 1, respectively. The binary values each indicate time preset for the buses 13a-13d to gate the TSL buffer 12 respectively. The state of the sequential operation based on the setting value of the time is shown in the timing chart of FIG. 2. Data from the data input terminal 6 is supplied to the register 5 for each clock pulse from the clock terminal 7 until the shift register 5 reaches the full state. In response to the next clock pulse supplied from the clock terminal 7, the data stored in the register 5 is sent to the latch circuit 8 in a bit-parallel fashion. In order to set the driving section 11 to the operable state during the selected cycle, the latch circuit 8 supplies information from the register 5 to the TSL buffer 12 in response to a latch clock signal supplied via the latch clock terminal and the lead wire connecting the latch circuit 8 and the latch clock terminal 10. The TSL buffer 12 supplies a signal from the control bus conductor 13 and a signal from the latch circuit 8 to the output transistor 15 so as to turn on/off the transistor, thereby blinking the LED element 3.
Next, the block circuit configuration of FIG. 3 will be described in conjunction with a printer system having the apparatus and the method of controlling the driving circuit described with reference to FIGS. 1-2. The printer system mainly includes two sections, namely, a printing head 1 and a control circuit 20. The printing head 1 is configured in the similar fashion to the printing head described above and hence will not be duplicatedly described. The control circuit 20 separately disposed with respect to the printing head 1 comprises a host computer 21 for transmitting necessary information or commands concerning texts or characters to be reproduced, a character generator 22 for generating a plurality of character signals of a position and a configuration in response to the information from the host computer 21, a first programmable read-only memory (ROM) 23 for supplying the shift register 5 with light emission signals to cause the printing element 2 such as the LED array head to emit light in response to the character signals from the character generator 22, and a second programmable ROM (PROM) 24 for supplying the first PROM 23 with information concerning emission light intensity characteristic of the printing element 2. The character signals generated from the character generator 22 cause the LED elements 3 to emit light according to the respective signals of the plural character signals. The light emission signals thereof are supplied from the first PROM 23 to the shift register 5. The first PROM 23 only outputs load information to the shift register 5 to cause a uniform light emission energy to be generated from each of the LED elements 3, namely, a so-called fixed PROM on which data erase and rewrite operations cannot be effected. The first PROM 23 is connected to the second PROM 24 to be supplied with light emission positional information. The second PROM 24 is a so-called variable PROM in which the data erase and rewrite operations are possible and is programmed so as to supply the first PROM 23 information related to the light emission intensity of the LED array head 3. Incidentally, the driving section 11 includes, as also shown in FIG. 1, the output transistors 15 and load resistors 16 of the output transistors 15.
The LED elements 3 constituting the recording element each possess a unique light emission intensity. However, in a case where the LED elements 3 are incorporated into a mechanism of the printing head for use, each LED element 3 must develop a uniform light emission output energy, which therefore requires correction means. The emission light output energy is represented by a product of a light emission intensity and a light emission time; consequently, if the light emission intensity is detected, the emission light output energy of each LED element 3 can be substantially fixed by controlling the light emission time. In the case of the example above, four data bits are provided to control the light emission intensity so as to construct a matrix together with the control bus conductor 13 commonly wired for the switch selection, thereby establishing 16-stage time intervals for the light emission. Conventionally, such a printer generally includes two sections, namely, a section of a printing head driving IC in which a shift register 5, a latch circuit 8, and a driving section 11 are integrated in a chip and a printing head 1 constituted from a printing element 2 including LED array elements and the like and a section of a control circuit 20 for controlling the printing element 2 via the driving IC 4.
Since the prior-art printer is constituted as described above, there have been problems that the control circuit occupies a great areal ratio in the printer system, which greatly increases the price thereof; moreover, that the head driving IC element integrated in a chip excepting the portion of the printing element 2 including the LED elements 3 is considerably large in size and the price thereof is hence increased. The large size and a high degree of integration of the head driving IC 4 are caused by a fact that in a case where the energy of light emitted from each LED element 3 is corrected through two stages, n shift registers and n latch circuits must be disposed for an LED element 3. Furthermore, since the correction on light emission from the LED head or the like is effected by selecting one of several stages of light emission time, a plurality of lead lines 9 for the respective bits must be used to send signals from the shift registers to the latch circuits, namely, the internal wirings are increased, which causes the IC to be of a large size.
Next, according to a printer to which the printing head above is applied, as the print speed becomes higher, variations in the size and position of each dot in a dot pattern are increased, which leads to a problem that the quality of the printed characters and image is lowered for the following reasons. First of all, as shown in the characteristic diagram of FIG. 2, in a case of a 15-stage correction of the emission light intensity, the period of time required for the driving circuit 4 to effect a correction on the LED element 3 varies in a rang from the minimum unit time to the maximum 15-unit time, namely, the maximum difference is represented by 15 times of the unit time. Particularly, in a printer using an array head in which a plurality of LED elements 3 are incorporated, a photosensitive drum for receiving light to form an image to be printed out is rotating at a constant rotary speed and hence a portion irradiated by the light is continuously moving. Consequently, if the period of time of the light irradiation is varied, the area irradiated by the light is also changed. That is, when the period of time required to scan the entire LED array head is sufficiently small as compared with the scanning period, the error does not become so great to cause a problem; however, in a case of a high-speed printing, the difference of the area irradiated by the light greatly varies and hence the problem cannot be ignored.